I've brought loads of parts and riding gear into Thailand over the years. Last time my bags got scanned by Thai customs and I had two bags full of parts and riding gear (mostly used, some new). I didn't get fined or anything but they told me not to do it again. I didn't even know it was a big deal until then.

 

I plan on taking a pair of motorcycle wheels with me out of Thailand, having them powdercoated, then bringing them back into Thailand. It's a short duration trip, I've got loads of unused baggage capacity, and my friend will do the work for free. What forms, etc. do I need to complete / submit when leaving Thailand so I am able to bring the wheels back and not have problems?

Except for the free. Why not simply get them powder coated here? Not ridiculously expensive. Might be less hassle.

Update:

I learned that "Vehicle parts" are classified as "restricted goods" (goods of which the imports and exports are restricted by law and therefore require a permit from related government agencies). In this case the issuing body of such a permit is the Ministry of Industry  (www.industry.go.th). No hint of a permit application on the site. Dead end.

 

So:
I packed the wheels into a suitcase.
I arrived at Suvarnabhumi and went to the outgoing customs (VAT) station located all the way to the right.
I explained the situation to the half-awake (or fully stoned) guy. He said I didn't need anything. I repeated the situation to him and offered to show him the wheels packed in the suitcase. Again, he said I didn't need any documents.
I flew out, flew back in.
I Arrived around midnight with a huge number of people.
I went through the green (nothing to declare) customs area. Nobody was working there.
Done!

Bottom line: Don't sweat it. Pack the parts in your checked baggage. Play dumb if, by some stroke of bad luck, the contents of your luggage are examined. The odds are in your favor.

 

Hopefully this is of help to someone